- Unrealized and Realized Gain/Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Natural gas commodity
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	$ 5,263	$ (17,555)	$ 17,347	$ (13,923)
Total loss in pre-tax earnings	(18)	(5)	(445)	(5)
Foreign exchange
Derivative [Line Items]
Amounts deferred to regulatory accounts on balance sheet	(216)	(267)	(224)	(201)
Total loss in pre-tax earnings	0	0	0	0
Benefit (expense) to cost of gas | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	(5,172)	16,710	(16,515)	14,074
Benefit (expense) to cost of gas | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	216	267	224	201
Operating gain (loss) | Natural gas commodity
Derivative [Line Items]
Gain (loss) on derivative, net	(109)	840	1,277	156
Operating gain (loss) | Foreign exchange
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 0	$ 0	$ 0